This invention relates to a method and apparatus for forming a sheet of material about a forming die to produce a sheet material part having a desired shape.
Numerous methods are known in the art for forming sheet material into complex contours and curved shapes. The working of sheet metal into complex or curved shapes is an area of art encompassing a great number of techniques some of which have been known to metal working artisans for centuries. More recently, processes have been developed to form sheet metal into complex shapes for a wide range of applications. The drop hammer process is used to press sheet metal between male and female dies to create contoured sheet metal parts. The Bag press forming process employs high pressure fluid to press a sheet metal workpiece against a die to produce a sheet metal part having a desired shape conforming to the die. Stretch forming is used to create contoured sheet metal parts such as aircraft skins by stretching sheet metal in relation to a male die until the sheet metal conforms to the desired contour. Explosive methods have been used to suddenly force sheet metal into a die with the pressure of an explosive charge. More recently, super plastic forming operations have been developed to form sheet metal into or over a die at high temperatures with a combination of gravity or the pressure of inert gas. All of these methods require specialized tooling and equipment.
Spinning is a very old sheet metal process for producing axially symmetric sheet metal parts. Spinning is illustrated in FIG. 1A and FIG. 1B. In the spinning process, a sheet metal workpiece is held in relation to a mandrel that defines the desired inside contour for the finished part. The outer surface of the mandrel is a surface of revolution. As can be seen in FIG. 1B, the part is formed by rotating both the mandrel and the sheet metal workpiece in unison while pressing the sheet metal against the mandrel with a forming tool. The path of the forming tool begins where the sheet metal workpiece is tangent to the mandrel and gradually proceeds along the mandrel until the sheet metal workpiece is formed to the mandrel. While the path of the forming tool relative to a fixed frame of reference is a simple curve in a plane, the path of the forming tool relative to the spinning mandrel and the workpiece is a spiral that traces the outside surface of the finished part. Spinning is a useful and effective process for producing parts defining a surface of revolution, however, spinning can not be used to shape parts that do not define a surface of revolution.
The present invention is a method for forming sheet material about a contoured die to produce a part having a contoured shape. In most applications the invention method is used to form sheet metal. The invention method forms material in a manner similar to the ancient process of spinning. However, where spinning uses a rotating workpiece and a rotating axially symmetric die, the invention method uses a moving forming tool to press a stationary workpiece against a stationary die to produce a part that does not need to have a shape that is an axially symmetric figure of revolution. The invention method includes these steps: (1) A thin walled part is defined to have a shape bounded by an inside surface and the outside surface. (2) A rigid die is fashioned having a forming surface corresponding to either the inside surface or the outside surface of the part. (3) A suitable sheet of workpiece material is held in a fixed relation to the die so that the material is tangent to the die at a tangent area. (3) A forming path is defined that follows the surface definition of the part that is opposite from the surface used to define the die. The forming path begins at a point adjacent to the tangent area and traces around the tangent area in circuits that incrementally offset away from the tangent area until all of the surface of the part is traced. (4) A forming tool is then moved along the forming path so that the contact surface of the forming tool presses the sheet material against the die thus gradually forming the sheet material to the shape of the die. When the forming tool reaches the end of the forming path, the sheet material is completely formed against the die and has taken on the desired shape of the finished part.
A digital computer can be used to define the inside and outside surfaces of the part and a digital computer can also be used to define the forming path. With such digital definition, it is possible to fashion a die using well known numerically controlled machining methods. By using a numerically controlled machine, it is also possible, using known methods, to move the contact surface of a forming tool along a forming path as described above. Accordingly, the invention process can be performed where digital part definitions and numerically controlled machines are available. Unlike with processes that rely on drop hammers and hydro-presses, with the invention method only a small amount of energy is used to form a workpiece at any one time so that forming dies for the invention process do not have to be made of strong materials that can withstand very large forces. Consequently, lighter, low cost and easily workable materials such as laminated wood can be used to fashion a forming die for the invention method. Moreover, only a single die is needed to make a contoured sheet metal part using the invention method. The invention method can be used to produce parts that would otherwise require the use of expensive specialized hammer mills and hydrodynamic presses. Because the invention method exploits widely available digital definition technology and uses standard numerically controlled machinery, it is now possible to make contoured sheet metal parts where they could not be previously made. With the invention forming process, it is also now possible to make such parts in small quantities where before the non-recurring costs of making small quantities in terms of machinery and tooling would have made it too expensive to produce such small quantities.